1. Field of the Invention
The present invention relates to a developing apparatus for developing an image with developing powder and more particularly to a developing apparatus capable of preventing scattering of the developing powder when the same developing powder is supplied.
2. Description of the Related Art
As a conventional developing apparatus which visualizes an electrostatic latent image by applying developing powder to the same electrostatic latent image, for example, a structure shown in FIG. 7 has been well known.
This developing apparatus includes a cylindrical developing powder bearing member 102, which rotates opposing an image bearing member 101, a first developing powder accommodating chamber 110 which is located adjacent to the developing powder bearing member 102, accommodates developing powder and supplementary developing powder supplied from a supplementary cartridge 103 located above, and agitates the developing powder with an agitation screw 104 and a second developing powder accommodating chamber 111 which supplies developing powder to the developing powder bearing member 102 with a conveyance screw 105, and the first developing powder accommodating chamber 110 and the second developing powder accommodating chamber 111 are partitioned by a partition wall 106.
As shown in FIG. 7B, the first developing powder accommodating chamber 110 and the second developing powder accommodating chamber 111 communicate with each other at both ends in a rotation direction of the screws 104, 105 and the agitation screw 104 and the conveyance screw 105 which are provided in the first developing powder accommodating chamber 110 and the second developing powder accommodating chamber 111, respectively convey developing powder in opposite directions to each other.
In such a developing apparatus, the supplementary developing powder charged to the surface of developing powder in the first developing powder accommodating chamber 110 from the supplementary cartridge 103 above the first developing powder accommodating chamber 110 is supplied to a position 107 indicated in FIG. 7B and traverses the first developing powder accommodating chamber 110 while being agitated by the agitation screw 104, so that it is charged receiving sufficient frictional electrification in this period. That developing powder flows into the second developing powder accommodating chamber 111 and is supplied to the developing powder bearing member 102 by the conveyance screw 105.
The developing powder supplied to the rotating developing powder bearing member 102 is conveyed into a developing area in the vicinity of the image bearing member 101 and developing powder is transferred to a latent image on the image bearing member 101 by a vibrating electric field so that the latent image is visualized. Developing agent not used for development passes the second developing powder accommodating chamber 111 and is conveyed to the first developing powder accommodating chamber 110 again. Then, circulation of the developing powder is continued so that toner and carrier are mixed within a certain range of uniformity with repeating the supply and agitation.
In such a developing apparatus, developing powder needs to pass the second developing powder accommodating chamber 111 accompanying agitation of the agitation screw 104 in a period since the developing powder is supplied to the first developing powder accommodating chamber 110 by the time when it reaches the developing powder bearing member 102. Particularly initial developing powder is charged sufficiently so that appropriate development can be carried out without fog with appropriately charged developing powder between the developing powder bearing member 102 and the image bearing member 101.
However, with an increase in process speed of copying machines in recent years, supplementary developing powder is scattered within the first developing powder accommodating chamber 110 by a motion of the agitation screw 104 in the conventional developing apparatus structure and in the worst case, the scattered developing agent reaches the image bearing member 101 thereby possibly causing fog. Further, the height of developing powder in the developing apparatus differs due to such environmental difference as temperature and humidity around the developing powder even if the weight of the developing powder is the same. Thus, when the height of the developing powder exceeds the height of the agitation screw 104, the supplementary developing powder moves just as if it slips on the surface of the developing powder without being fetched into the inside of the developing powder and agitated well, and consequently, such a problem that developing powder charged low is conveyed to the developing agent bearing member 102.
In view of the problem with scattering of developing powder inside the developing apparatus, Japanese Patent Application Laid-Open No.2003-107859 has taken a measure for reducing a reaction force generated when developing powder is exchanged between the screws and Japanese Patent Application Laid-Open No.2003-215917 has taken a measure of preventing the scattering of developing powder to the outside of the developing apparatus with an end seal.
Additionally, other countermeasures such as increasing the height of the partition wall 106 which partitions the first developing powder accommodating chamber 110 and the second developing powder accommodating chamber 111 or increasing the quantity of agitation members, called ribs, provided on the side face of the agitation screw 104 have been taken.
However, if considering acceleration of process speed in an image forming apparatus such as a copying machine in recent years, no remarkable effect to the scattering of developing powder in the developing apparatus can be expected even if the height of the wall face is intensified or the quantity of the ribs is increased according to the above-described conventional methods and as a consequence, there is a fear that developing powder charged by insufficient friction may adhere to the developing powder bearing member.